DUBAI/DOHA (Reuters) - Arab states that have cut diplomatic and transport ties with Qatar tightened their squeeze on Friday by putting dozens of Qatar-linked people on terrorism blacklists, while Qatari ally Turkey came to its side with plans to send troops, warships and planes.

The developments intensified a confrontation between tiny-but-wealthy Qatar and a group of Arab nations led by Saudi Arabia and Egypt that accuse it of fomenting instability. The dispute has created a major diplomatic test for the United States, which is close allies of the countries on both sides.

In an apparent escalation of the crisis, staff at Al Jazeera, Qatar's influential satellite television news channel which often infuriates the rulers of the Arab world, said on Thursday its computer systems were under cyber attack.

Riyadh, Cairo and their allies accuse Qatar, the world's richest country per capita, of supporting militant Islamist movements across the region. They have imposed what Qatar says is a blockade of shipping and air traffic, and closed Qatar's only land border, causing panic buying at supermarkets.

Assertive foreign policy

Qatari Foreign Minister Sheikh Mohammed bin Abdulrahman bin Jassim Al-Thani attends a meeting with Russian Foreign Minister Sergei Lavrov in Moscow Thomson Reuters Qatar, which has developed an assertive foreign policy over the past decade, denies that it supports militants and says it is helping to reduce the threat of terrorism by backing groups that fight poverty and seek political reform.

Foreign Minister Sheikh Mohammed bin Abdulrahman al-Thani called the moves by Arab neighbors and others "clear violations of international law and international humanitarian law.

"They will not have a positive impact on the region but a negative one," the minister said during a visit to Germany.

German Foreign Minister Sigmar Gabriel called for diplomatic efforts to resolve the worst crisis to grip the region in years.

"We are convinced that now is the hour of diplomacy and we must talk to each other; along with our American colleagues but above all our colleagues in the region, we must try to find solutions, especially lifting the sea and air blockades."

Washington relies closely on the countries on both sides of the dispute for its military operations in the Gulf: Qatar hosts the U.S. Air Force's biggest base in the region, while Bahrain hosts the U.S. Navy. Both Saudi Arabia and Qatar have funded rebels fighting in the Syrian civil war.

The conflagration erupted just weeks after U.S. President Donald Trump visited Saudi Arabia. Trump initially responded by tweeting his support for moves against Qatar, even as his State Department and Defense Department sought to remain neutral.

With supply chains disrupted and anxiety mounting about economic turbulence, banks and firms in Gulf Arab states were trying to keep business links to Qatar open and avoid a costly firesale of assets. The riyal currency has tumbled and the cost of insuring Qatari debt against default has risen.

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Clashing over brotherhood

Saudi Arabia, the United Arab Emirates (UAE), Egypt and Bahrain added 59 people to terrorist blacklists, among them 18 Qataris, including Abdullah bin Khalid Al Thani, a former interior minister and member of Qatar's royal family.

The Qatari government said the move "reinforces baseless allegations that hold no foundation in fact".

"Our position on countering terrorism is stronger than many of the signatories of the joint statement - a fact that has been conveniently ignored by the authors," it said in a statement. Those on the list, including the former interior minister, could not be reached for comment.

Many of the others added to the list are figures associated with the Muslim Brotherhood who have made Qatar a base, including Brotherhood spiritual leader Yousef al-Qaradawi. Some are prominent jihadists who have fought in Libya and Syria.

Qatar has angered its neighbors for years by supporting the Brotherhood, a decades-old underground movement that calls for rule based on Islamic principles.

The Brotherhood says it eschews violence but some Arab states call it a terrorist movement. It came to power in Egypt in an election in 2012 but was toppled a year later by the military.

Turkish President Tayyip Erdogan, whose AK Party has its roots in Islamist politics and who has voiced support for the Brotherhood, signaled his firm backing for Qatar by swiftly signing a law to send Turkish troops to a base there.

In a signal of Turkey's urgency over the issue, parliament passed the law on Wednesday, Erdogan signed it on Thursday and it was published in the state gazette by Friday.

Turkish warplanes and warships

People shout slogans as they hold Turkish and Qatari flags during a demonstration in favour of Qatar in central Istanbul, Turkey, late June 7, 2017. Reuters/Murad Sezer Turkey will send warplanes and warships to Qatar after an initial deployment of troops at a base in Doha, the mass-circulation Hurriyet newspaper said on its website.

"The number of Turkish warplanes and Turkish warships going to the base will become clear after the preparation of a report based on an initial assessment at the base," Hurriyet said. Around 90 Turkish soldiers are currently at the base, it said.

Turkish officials were not immediately available to comment on the report but Hurriyet said there were plans to send some 200-250 soldiers within two months in the initial stage.

Staff at Al Jazeera said on Thursday the Qatari state-funded network, which is watched by millions of people across the Arabic-speaking world, had come under a sustained cyber attack but was still functioning.

Qatar also said last month its state news agency had been hacked and false statements attributed to the country's ruler posted, helping ignite the current rift with other Arab states.

Qatar said it leads the region in attacking what it called the roots of terrorism, giving young people hope through jobs, educating hundreds of thousands of Syrian refugees and funding community programs to challenge extremist agendas.

Saudi Arabia, Egypt, the UAE and Bahrain severed relations with Qatar on Monday, accusing it of supporting both Sunni Muslim Islamist militants and Shi'ite Muslim Iran - charges Qatar rejects. Several other countries later followed suit.

Would-be mediators, including Trump and Kuwait's ruling emir, have struggled to ease the crisis.

Trump initially took sides in tweets with the Saudi-led group, before apparently being nudged into a more even-handed approach when U.S. defence officials renewed praise of Qatar where their base is located.

Qatar's ambassador to Washington said on Thursday his government trusted Trump's ability to resolve the dispute.

"The most important engagement that happened so far from the U.S. is by the president, which we highly appreciate," Meshal bin Hamad al-Thani told the Financial Times. "We truly believe that the involvement of the president and the U.S. will bring this crisis to an end."

The ambassador left open the prospect of compromise, saying: "We are courageous enough to acknowledge if things need to be amended."

(Additional reporting by William Maclean and Rania Gamal in Dubai, Andrea Shalal in Germany; Writing by Peter Graff, editing by Peter Millership)